Microlithographic processes are commonly used in the manufacture of miniaturized devices, such as integrated circuits, liquid crystal elements, micro-patterned structures and micro-mechanical components. In such processes, a projection objective projects patterns of a patterning structure (usually a photo mask (mask, reticle)) onto a substrate (usually a semiconductor wafer). The substrate is coated with a photosensitive layer (resist) which is exposed with an image of the patterning structure using projection radiation.
Generating a new design of a projection objective can be a complicated task involving an optimization of structural parameters and quality parameters of the projection objective. The structural parameters include refractive indices of materials of which the lenses are formed, surface shape parameters of lenses and mirrors (if applicable), distances between first and second surfaces of each lens, distances between surfaces of different optical elements, a distance between the object plane of the projection objective and an entry surface of the object-side front element of the projection objective, a distance between an exit surface of an image-side front element of the projection objective and the image plane, refractive indices of media disposed between adjacent optical elements, between the object plane and the object-side front element and between the image plane and the image-side front element. Quality parameters include parameters describing the optical performance of the projection objective e.g. in terms of selected aberrations, image-side numerical aperture, magnification of the projection objective and the like.
The optimization of a design to conform to a desired specification of the optical performance and other quality features of the projection objective typically involves computational methods such as ray tracing to optimize the parameters of the projection objective while observing certain boundary conditions. CODE V, a lens analysis and design program sold by Optical Research Associates, Inc., is a commonly used software tool employed for that purpose. The optimization includes minimizing or maximizing a suitably chosen merit function depending on the parameters of the design. Typically, the merit function construction is done by utilizing several merit function components, which may represent optical aspects, manufacturability aspects and other aspects describing the optimization goal of the specific design.